Implantable medical devices include, among other things, cardiac function management (CFM) devices such as pacers, cardioverters, defibrillators, cardiac resynchronization therapy (CRT) devices, as well as combination devices that provide more than one of these therapy modalities to a subject. Such devices often include one or more diagnostic capabilities. Moreover, such diagnostic capabilities may be used as a basis for automatically providing therapy to the subject or for communicating diagnostic information to a physician or to the subject.
One example of a diagnostic capability is sensing intrinsic electrical heart signals. These intrinsic heart signals include depolarizations that propagate through heart tissue. The depolarizations cause heart contractions for pumping blood through the circulatory system. The intrinsic heart signals are typically sensed by an implantable medical device at implanted electrodes. The implantable medical device typically includes sense amplifier circuits and other signal processing circuits to extract useful diagnostic information from the intrinsic heart signals.
A different example of a diagnostic capability is sensing an interelectrode impedance, that is, detecting an impedance between electrodes. Such electrodes typically include, among other things, electrodes that are implanted in a subject's thorax. Electrodes that are implanted in a subject's thorax may include, among other things, “intracardiac” electrodes located within the subject's heart. Another example of thoracic electrodes includes intravascular electrodes located in the subject's vasculature. A further example of thoracic electrodes includes epicardial electrodes that are located on an outer surface of the subject's heart. Yet another example of thoracic electrodes includes housing electrodes that are located on a typically hermetically sealed “can” of a pectorally or abdominally implanted CRM device electronics unit, or on an insulating “header” of such an electronics unit.
A tissue impedance between electrodes is typically obtained by introducing a test current into the tissue and sensing a responsive voltage between two electrodes (or vice-versa). The electrodes used for introducing a test current or test voltage need not be the same electrodes as those used for respectively measuring the responsive voltage or responsive current.
An impedance signal obtained between two intracardiac electrodes will be affected and modulated by, among other things, the subject's heart contractions and the subject's breathing. These two impedance-derived signals are sometimes referred to as the cardiac stroke signal and the respiration signal, respectively. Each provides useful diagnostic information. For example, the cardiac stroke signal may be used as an input variable to responsively adjust a pacing rate or another parameter of a “stroke volume” or other cardiac pacing therapy algorithm. Similarly, the respiration signal (e.g., amplitude, frequency, etc.) may be used as an input variable to responsively adjust a pacing rate or another parameter of a “minute ventilation” or other cardiac pacing therapy algorithm. In sum, the impedance-derived cardiac stroke and respiration signals can provide useful diagnostic information for a cardiac rhythm management device.